The main function of the lungs is to provide continuous gas exchange between inspired air and the blood in the pulmonary circulation, supplying oxygen and removing carbon dioxide, which is then cleared from the lungs by subsequent expiration. Survival is dependent upon this process being reliable, sustained and efficient, even when challenged by disease or an unfavourable environment.
Lung function tests evaluate how much air lungs can hold, how quickly air moves in and out of the lungs, and how well lungs add oxygen to and remove carbon dioxide from the blood. Such tests can help diagnose lung diseases and measure the severity of lung problems that prevent normal breathing.
Lung function tests are done to:                Help determine the cause of breathing problems;        Measure the amount of lung function in a person who has a lung disease and monitor the effectiveness of treatment;        Identify people at high risk of developing lung disease (especially smokers);        Evaluate a person's ability to breathe before surgery;        Monitor the lung function of a person who is regularly exposed to substances that can damage the lungs.        
Several different types of tests can provide information about lung function. Such tests include spirometry, gas dilution tests, body plethysmography, carbon monoxide diffusing capacity and arterial blood gases.
Spirometry measures the volume of air inspired or expired as a function of time and is the standard method for measuring most relative lung volumes; however, it is incapable of providing information about absolute volumes of air in the lung. Thus a different approach is required to measure residual volume, functional residual capacity and total lung capacity.
Two methodologies most commonly used for determination of absolute lung volume are gas dilution and body plethysmography.
Gas dilution tests measure the amount of air that remains in the lungs after the subject has exhaled as completely as possible (residual volume). Body plethysmography measures the total amount of air that lungs can hold.
During body plethysmography, the subject sits in an airtight box (body plethysmograph, or ‘body box’) of known volume and breathes through a mouthpiece connected to a shutter. The pressure is monitored in two places, in the box and at the subject's airways, the latter via a side-port of the mouthpiece. At end-expiration the airways are momentarily occluded by the shutter, and the subject makes an inspiratory effort against the occlusion. The increase in their chest volume slightly reduces the box volume whilst slightly increasing the pressure in the box.
Monitoring changes in pressure in the box and applying a series of well documented derivation techniques, body plethysmography allows a number of pulmonary measurements to be obtained such as for example thoracic gas volume and airways resistance.
Drawbacks associated with body plethysmography:                Mental confusion, muscular incoordination, body casts or other conditions that prevent the subject from entering the plethysmograph cabinet or adequately performing the required manoevours (i.e. panting against a closed shutter);        Claustrophobia may be aggravated by entering the plethysmography cabinet;        Presence of devices or other conditions such as continuous I.V infusions with pumps or other equipment that will not fit into the plethysmograph that should not be discontinued, or that might interfere with pressure changes (eg. chest tube or ruptured eardrums);        Continuous oxygen therapy that should not be temporarily discontinued;        Over estimation of thoracic gas volumes in subjects with severe obstruction or induced bronchospasm unless a slow ‘panting’ speed is maintained;        Erroneous measurement of thoracic gas volume, airways resistance, or specific airways conductance due to improper panting technique. Excessive pressure fluctuations or signal drift during panting may invalidate thoracic gas volume, airways resistance or specific airways conductance;        Whole-body plethysmographs are expensive and usually found in pulmonary function laboratories, cardiopulmonary laboratories, clinics and specialist pulmonary offices.        
An object of the present invention is to provide a cost effective and easy to use, pulmonary evaluation device which does away with the requirement of the patient having to be placed in an air-tight box so that, for example, it can be used at the General Practice (GP) level.
A further object of the invention is to provide a pulmonary evaluation device that can be used by a variety of subjects or users. Such subjects including neonatal, paediatric, geriatric, disabled, the mentally frail and animals.